DESCRIPTION: (Applicant?s Abstract) This proposal describes the development of probes for three-dimensional imaging of retinal activity. These probes are magnetic resonance imaging (MRI) contrast agents that are sensitive to membrane potentials. It is the signal from water protons that provides the image in typical MRI images and contrast agents interact with water to enhance that signal. In our previous work we have shown that by blocking the interaction of water with the contrast agent we can "turn off? enhancement, and then selectively "turn on" enhancement by removing the block. In these previous experiments we added a blocking group to the contrast agent that is removable by enzyme cleavage, so that signal enhancement was dependent upon enzyme cleavage. Based on these results we hypothesize that we can modulate the ability of an agent to enhance MRI signal by adding a group that is sensitive to membrane potentials, thus making signal enhancement dependent on membrane activity. In our research design we propose to: 1) Design and synthesize an agent based on a known optical indicator of membrane potentials 2) Characterize the ability of the new agent to enhance contrast in cell-free and cell culture systems. 3) Perform studies leading to the application of the new agents in eyes in vivo. These studies include determining the optimal route of delivery for the agent, and its biodistribution in model animal studies. These investigations will ultimately be extended to rodent models of retinal degeneration to map the loss of retinal activity over time and correlate the results with those observed by traditional methods. Results of the proposed studies will provide a noninvasive method to image membrane activity in deep tissues; this may aid in the early diagnosis of retinal disease. While our ultimate goal is to apply these agents in studies of macular degeneration, these agents would be useful for observations of any system in which membrane signaling activity plays a role.